Instant photography is well known in which transfer dye images are instantly formed utilizing the color diffusion transfer process. In this instant photography, there is generally used a photographic film unit including at least one silver halide emulsion layer on a support, said emulsion layer containing dye-providing compounds, and an image-receiving layer to fix therein diffusible dyes as color images, which is provided on the same support as above or a different support, and an alkaline aqueous processing solution is extended between the two sheet-like photographic elements (i.e., a light-sensitive element and an image-receiving element, or a light-sensitive element and a cover sheet) to thereby form dye images. In addition, a tank development type material comprising a single sheet as described in Japanese Patent Application (OPI) No. 119345/82 (the term "OPI" as used herein means a "published unexamined Japanese patent application") is of course known.
The most important of requirements for these diffusion transfer photographic light-sensitive materials, as can be readily understood from the term "instnat photography", is that the desired image can be formed in very short periods of time. When, for example, the formation of images is intended to be completed within 1 minute, it is necessary for the development of silver to be completed in as short a time as possible, i.e., within the time of from 10 to 15 seconds, because a considerably long time is needed for the diffusion (or release) of image-forming substances following the silver development.
It is well known that high-speed development increases fog, resulting in a deterioration in discrimination (Ag.degree.max/Ag.degree.min ratio). Moreover, as is well known, the diffusion transfer process suffers from a serious disadvantage that fog is markedly increased since it uses a 1N or more alkaline processing solution. Such an abnormally high degree of fog is a characteristic problem of the diffusion transfer process in that the development must necessarily be achieved at high speeds incomparable with those for the conventional light-sensitive materials, and also the development must necessarily be performed under high alkalinity conditions which are not involved in the conventional light-sensitive materials. The term "fog" as used herein means a so-called fog ascribable to silver halide but not a "stain".
Such an increase in fog exerts the following adverse influences: when dye-providing compounds forming an imagewise image relative to a silver image (hereinafter referred to as "negative dye-providing compounds") are used, Dmin is high and the white ground is contaminated, whereas when dye-providing compounds forming a reverse imagewise image relative to a silver image (hereinafter referred to as "positive dye-providing compounds") are used, Dmax drops. The case is the same with the silver salt diffusion transfer process.
It has thus been strongly desired to prevent the formation of fog in the diffusion transfer process.
Various techniques have been proposed for the prevention of fog, including a method using such compounds as 1-phenyl-5-mercaptotetrazole as described in U.S. Pat. No. 3,265,498, a method using blocked development inhibitors as described in U.S. Pat. Nos. 3,698,898, 4,009,029, West German Patent Application (OLS) No. 2,427,183, U.S. Pat. No. 3,265,498, etc., and a method using mercapto compounds as described in U.S. Pat. Nos. 4,355,092 and 4,355,101. These methods, however, fail to attain the object satisfactorily. That is, although they permit the prevention of fog, an undesirable problem arises in that the rate of development is seriously reduced, while on the other hand when the rate of development is less seriously reduced, the effect of fog prevention is small. This is prominent especially for cases in which a silver halide emulsion is a negative meulsion forming a surface late image. The reason for this is believed to be that the negative emulsion is especially readily fogged because of its surface sensitivity and, furthermore, the rate of development of fog is high.
Moreover, the conventional fog-preventing compounds undesirably decrease sensitivity.